1. Field of the Invention
The present invention relates generally to methods for removing contaminants from water and soil. More particularly, the present invention involves the treatment of groundwater and/or soil with bacteria under anerobic conditions to remove perchlorates and/or nitrates.
2. Description of Related Art
The publications, patents and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.
Perchlorate (ClO.sub.4) is an oxyanion that has been used extensively in the chemical and aerospace industries because it can act as a strong oxidizing agent. Ammonium perchlorate has been used in solid rocket fuel, explosives and pyrotechnics. The mishandling of perchlorate at aerospace-related industrial sites is the likely source of perchlorate that has recently been discovered in surface and ground waters. The evident persistence of perchlorate in the environment and its toxicity to humans at sufficient concentrations has raised concern over drinking water quality standards and possible environmental impacts. Perchlorate is not currently regulated under the Safe Drinking Water Act, although the California Department of Health Services has established an action level of 18 .mu.g L.sup.-1 based on an evaluation of toxicity data by the Environmental Protection Agency. Perchlorate has been found in certain drinking water wells in California and Nevada at concentrations that exceed this action level. In response, the California Department of Health Services has advised that water from these wells should not be used as a source of drinking water.
The development of effective and efficient strategies for the remediation of the perchlorate found in groundwater is an area of intense interest (7). Remediation strategies for the removal of perchlorate based on adsorption by activated carbon have not proven to be highly efficient due to rapid saturation of perchlorate adsorption sites. Other advanced procedures for the removal of perchlorate include reverse osmosis and ion exchange. However, these purification processes are very expensive. In addition, the preceding systems are limited due to the fact that once the perchlorate has been removed from groundwater a second disposal strategy is required to deal with the perchlorate that has been collected.
There also has been intense interest in utilizing microorganisms to remove perchlorate from water by biological processes. The degradation of perchlorate utilizing biological systems provides an attractive remediation strategy for a variety of reasons. For example, microorganisms which degrade perchlorate can completely transform perchlorate into an innocuous end-product, namely, chloride (1, 10, 13). Further, biological treatment can be used for the simultaneous treatment of perchlorate and nitrate. Nitrate pollution of groundwater can occur in agricultural regions, and the allowable concentration in potable water sources is strictly regulated. Finally, biological treatment processes can be quite cost effective when compared to the more expensive physical and chemical processes.
The transformation of oxyanions, such as perchlorate and chlorate, occurs in the absence of oxygen as the result of anaerobic respiration. Microbial respiration couples the oxidation of an organic substrate, such as glucose or acetate, to the reduction of a final electron acceptor, usually oxygen. Under anaerobic conditions, the oxidation of organic compounds requires the use of an alternative electron acceptor in place of oxygen, such as nitrate (NO.sub.3), manganese (Mn[IV]), iron (Fe[III]), or sulfate (SO.sub.4). Bacteria capable of anaerobic respiration are common to soil and sediment environments where anaerobic conditions are prevalent and natural sources of alternate electron acceptors are common. As a highly oxidized compound (+7 oxidation state), perchlorate has a high potential for utilization as an alternate electron acceptor. Perchlorate-reducing bacteria which have been described in scientific literature include Vibrio dechloraticus Cuznesove B-1168 (8, 14), Wolinella succinogenes HAP-1 (21), and a proteobacteria (strain GR-1) described by Rikken et al. (13). Bacteria capable of reducing chlorate are also known. Examples include Ideonella dechloratans (11) and an Acinetobacter sp. (16).
The proposed pathway of anaerobic perchlorate reduction is as follows (13). ##STR1##
Several patents have described microbial-mediated reduction of perchlorate as a means of removing perchlorate from industrial waste water (2, 8, 23). A United States patent issued to Yakovlev et al. (23) describes the use of unaerated sewage sludge for the treatment of certain oxygen-containing inorganic chlorine and metal compounds, including perchlorate, chlorate and chromate. Domestic sewage sludge is mixed with contaminated wastewater and placed in a large tank. In the absence of aeration, microbial utilization of organic material within the sludge will rapidly deplete the available oxygen. Under anaerobic conditions, the reduction of oxygen-containing inorganic compounds occurs with the oxidation of organic compounds. Following the anaerobic phase, a second stage in the process removes the sludge from the water by precipitation. It is important in this process to supply an excess quantity of organic material, as measured by biochemical oxygen demand (BOD), in order to ensure the creation of an anaerobic environment. The patent also states that the BOD must be greater than the amount of oxygen in the form of inorganic oxygen-containing contaminants present in the wastewater.
Several later patents have improved on the above basic process by enhancing the rate and extent of perchlorate reduction and enabling the treatment of higher concentration of perchlorate in the wastewater. Korenkov (8) described a method of reducing perchlorate and chlorate under anaerobic conditions utilizing the bacterium, Vibrio dechloraticans Cuznesove B-1168. This organism is capable of reducing perchlorate and chlorate when grown anaerobically on acetate or ethanol as a carbon source (14). The authors reported reduction rates of perchlorate as high as 70 mg ClO.sub.4.sup.- per hour per gram biomass solids (dry weight), and the ability to treat perchlorate concentrations as high as 3 mM (about 300 mg L.sup.-1).
Another United States patent (2) describes a process in which contaminated water is added to an anaerobic bioreactor and spiked with a mixed bacterial culture. The bacterial culture contains a specific bacterium, Wolinella succinogenes, which was isolated from domestic sewage sludge for its ability to reduce very high concentrations (&gt;7000 mg/L) of perchlorate (1, 19, 21). High protein organic nutrients were found to support perchlorate reduction. The source of this oxidizable organic matter in the anaerobic bioreactor could be in the form of aged brewers yeast, cottonseed protein or whey powder. A second stage in the process removes nutrients and organic matter to improve the quality of the water for discharge. One advantage of this system is that it does not utilize sewage sludge, and therefore eliminates problems associated with the presence of pathogens. The bacterium isolated was capable of reducing perchlorate concentrations 26-fold greater than in previous reports, and was reported to have a specific perchlorate degradation rate of at least 1492 mg ClO.sub.4.sup.- per hour per gram biomass (dry weight). Through the use of a specific isolate and by optimizing nutrient and environment conditions, the anaerobic reactor was capable of greater perchlorate reduction rates than previously reported.
Wallace et al. (19) described the use of an up-flow anaerobic fixed-bed reactor containing a consortium of facultative anaerobic microorganisms including W. succinogenes HAP-1. The 1.2 m (length).times.7.6 cm (internal diameter) reactor could reduce 1500 mg per liter perchlorate to less than 100 mg per liter at a rate of 1 g perchlorate per hour per liter. Bioreactor systems have been optimized to treat high perchlorate concentrations (grams per L range), but there is currently little information about treating levels of perchlorate that are less than 1 mg per liter which can occur in subsurface water supplies. The primary objective of a groundwater treatment system is the removal of perchlorate to less than detectable levels.
The focus of the above research has been the remediation of perchlorate at the very high concentrations which are associated with wastewater generated in industrial situations, such as the manufacturing of solid rocket propellant. An additional problem which is of current concern is the presence of perchlorate at moderate to very low concentrations in subsurface water supplies used as a source of drinking water. To be an effective remediation strategy, biological transformation should ensure the reduction of perchlorate to concentrations less than the current action level of 18 mg L.sup.-1, and preferably, to below the limit of detection.
Nitrate is another groundwater contaminant that is commonly found in ground waters of agricultural regions. The U.S. Environmental Protection Agency (EPA) has set a limit of 10 mg NO.sub.3.sup.- -N per liter for potable water. Nitrate reduction can occur under the same conditions as perchlorate reduction, and the treatment of nitrate contaminated water using anaerobic bioreactor systems is a well established technology (3, 4, 15). Commercialscale bioreactor systems in operation in Europe provide an efficient removal of nitrate and can operate at a high capacity, with some systems able to treat up to 400 m.sup.3 per hour (5, 12). It would be desirable to apply similar technologies to the removal of perchlorate from groundwater. The simultaneous removal of perchlorate and nitrate within the same bioreactor would be a desirable feature of any groundwater treatment system.
As is apparent from the above, there is a present and continuing need to develop new processes and systems for removing perchlorate and nitrate from water.